The continuing drive for miniaturization of semiconductor devices has caused an increased rigor in the photolithography used to delineate the fine patterns of those devices. The demands for finer resolution have caused the shrinkage of imaging wavelengths from 365 nm (high pressure mercury lamp) to 248 nm (KrF excimer lasers), to 193 nm (ArF excimer lasers) and beyond. As the patterns and wavelengths become finer, the materials properties of the photoresists used for pattern delineation have become more and more demanding. In particular, requirements of sensitivity, transparency, aesthetics of the image produced, and the selectivity of the resists to etch conditions for pattern transfer become more and more strenuous. Because of this, the traditional lithographic materials, such as novolaks, diazonaphthoquinones, etc., are unsuitable platforms for ultra large-scale integration (ULSI) manufacture and beyond.
Advanced photoresists usually employ a technique called chemical amplification in which an acid generated by photolysis catalyzes a solubility switch from alkali insoluble to alkali soluble by removal of an acid sensitive group protecting an alkali-solubilizing moiety. The principle of chemical amplification as a basis for photoresist operation has been known for some years (see U.S. Pat. No. 4,491,628). Most chemically amplified resists have been designed around the use of acid sensitive carboxylic esters or acid sensitive hydroxystyrene derivatives.
The most common type of photoresists are called “single layer” photoresists in which the photoresist has both the function of imaging and plasma etch resistance. Another approach to solving the need for high etch resistance involves the use of multilayer resist systems, typically a bilayer system. In this approach, a thin photoresist imaging layer is deposited over a thicker planarizing layer (undercoat). The photoresist layer is assigned the function of imaging and the undercoat is assigned the function of plasma etch resistance. Bilayer photoresists typically contain silicon and this provides certain advantages in resolution from using thinner imaging films. In many cases the bilayer photoresist/undercoat stack provides better substrate plasma etch resistance than a single layer photoresist. The bilayer system is image-wise exposed and developed to uncover portions of the undercoat. Both layers are exposed to an oxidative etch typically with a gas comprising oxygen. The silicon in the bilayer resist oxidizes to silicon dioxide and protects the underlying undercoat. The uncovered undercoat is oxidized away and thus the image pattern in the resist is transferred into the undercoat. Examples of bilayer photoresists can be found for example in U.S. Pat. No. 6,359,078, U.S. Pat. No. 5,985,524 and U.S. Pat. No. 6,028,154, U.S. Pat. No. 6,146,793, U.S. Pat. No. 6,165,682 each of which is incorporated herein in their entirety.